With the rapid expansion of the markets for notebook personal computers, cellular phones, electric vehicles and the like, electric energy storage devices such as capacitors and secondary batteries are actively investigated. Among others, lithium ion secondary batteries are attractive because of being capable of storing larger amounts of energies. Currently, batteries having further higher energy densities are demanded, and as candidates meeting such demands, the use of metals such as tin and silicon, or the alloys and the oxides of these as negative electrode active materials is proposed. However, batteries having the higher energy densities are the larger in the releasable energies, and hence require the more thoughtful considerations about the safety.
For example, in electric energy storage devices such as lithium ion secondary batteries, separators prevent the short-circuit between the positive electrode plate and the negative electrode plate, and play a role of effectively migrating the lithium ions. Polyolefin-based microporous separators made of polypropylene materials or polyethylene materials have hitherto been mainly used. This is because the shut-down effect possessed by these materials contributes to the safety at the time of heat generation of batteries. However, the use of a polyethylene material for a high energy density battery results in the melting of the separator before the shut-down effect is obtained, and leads to a possibility of the occurrence of short-circuit between electrodes over wide areas.
It has been investigated to use, as a separator substrate high in heat resistance and high in strength, a substrate made of an inorganic material, in particular a substrate made of a woven fabric, and more particularly a glass cloth. Glass cloth is high in strength, and the melting point of nonalkali glass (E-glass) for materials for electronic boards is as high as 800° C. or higher. Glass cloth has no shut-down effect possessed by polyolefin-based microporous separators, but is predominantly higher in melting point as compared with organic polymers, and hence is sometimes rather excellent in safety.
Glass cloth is a product obtained by weaving yarns prepared by bundling glass fibers, and hence cutting processing of glass cloth sometimes causes texture collapse starting from the cut portion to be frayed, or glass cloth is sometimes deformed in the assembly steps of electric energy storage devices. When glass cloth is subjected to a force perpendicular to surface of the glass cloth, the texture is expanded to result in a possibility that the positive electrode plate and the negative electrode plate are short-circuited to each other. Accordingly, in order to reinforce glass cloth, a composite material composed of glass cloth and an organic polymer material, in particular a thermoplastic polymer material has been investigated.
Patent Document 1 (JP H10-12211 A) discloses a composite film for battery separators prepared by laminating a woven or non-woven fabric made of glass fibers having specific physical properties on a polyolefin microporous film made of a polyolefin having a specific high molecular weight. The lamination treatment is performed by a conventional calendering treatment.
Patent Document 2 (JP 4831937) describes a separator in which glass cloth is used as a substrate, and a layer made of an organic polymer is integrated as a binder with the front and back sides of the substrate. As the organic polymer, for example, polyvinylidene fluoride, polyvinylidene fluoride copolymer or polyvinylidene fluoride is quoted as an example. In this way, the reinforcement and the overcharging prevention function are imparted.
Patent Document 3 (JP 2004-269579 A) discloses, as a microporous film suitable as a separator for a lithium battery, a glass fiber fabric-reinforced polyolefin microporous film obtained by a process in which a film layer formed from a composition composed of a polyolefin and a solvent is superposed on and pressed to a glass fiber fabric, so as for the glass fiber fabric to be impregnated with the composition, and subsequently the impregnated glass fiber fabric is cooled and then the solvent is removed from the fabric concerned to produce an intended microporous film.
Also for lead storage batteries, separators including glass fiber fabric have also been investigated from the viewpoint of the mechanical strength. In the separators for lead storage batteries, glass fiber fabric is used as the supporting material to support the fibrous layer absorbing and immobilizing the electrolytic substance (electrolyte). As a separator for closed type lead storage batteries, Patent Document 4 JP 2005-503652 A) discloses glass fiber as the fibrous layer absorbing and immobilizing the electrolytic substance (electrolyte), and also discloses glass fiber fabric as the supporting layer of the fibrous layer.